Kustaa Nyholm wrote:
> would a 2nd interface require an other set of end points separate
> from the keyboard-HID interface?
Each interface has it's own endpoints, and endpoint 0 is considered
not belonging to any specific interface so it can be considered
common, but Windows may not allow useful access to it without say
WinUSB being installed.
Are the Microsoft descriptors an alternative? Ie. that one or both
interfaces use WinUSB when plugged in, and thus accessible using
libusb? Doesn't work on older Windows systems though. I clearly
assume that you are working with Windows, since anywhere else I think
it would be easier to use vendor specific.
> The hardware only supports four end points and two are already in
> use by the HID
FWIW a HID class interface isn't required to have an interrupt out
endpoint, so if the hardware has no further restrictions (e.g.
2/2 IN/OUT endpoints) then you can use just one endpoint for each.
> I'm skating on thin ice here,
Well, check, otherwise noone can give any useful advice.
> So we would need to use the existing HID out end point for our
> control transfer, or what?
That sounds confusing.
HID1_11.pdf Section 4.4 specifies the HID class interface, although
with sloppy terminology. They have a column with heading Pipe which
lists one pipe and two endpoints. Anyway, ep0 is common and given,
and not actually part of the interface at all beyond that it can also
carry requests that are destined for an interface.
HID class then specs one interrupt in endpoint which is required, and
one interrupt out endpoint which is optional.
So if the out endpoint exists it must be interrupt, so there are no
control transfers going over it.
A control transfer goes over a control endpoint, usually the only
control endpoint in a device is ep0, since it can carry also
interface-specific requests.
Endpoint 0 can certainly be used to send (small amount) data to the
device, but you must investigate exactly how and what you can
accomplish through the kernel drivers and system APIs that you are
able to use. WinUSB and libusb would make it trivial. HID makes it
more complicated. I understand you can't install but if target
platform is newer systems then perhaps you can still use WinUSB and
libusb by adding Microsoft descriptors to the device.
//Peter
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